The present invention relates to a circuit arrangement for a repetition-blocking device for preventing prohibited or undesired repetition of operations. This problem arises in a great number of technical fields. In chemistry, for example, it may be necessary during mixing processes not to permit the repeated addition of a given substance until another substance has been added.
Another field of application is the prevention of prohibited repetition of operations. As a well-known example, there may be cited self-service filling pumps. Here, misapplication is possible by illicitly reusing the filling pump after the first refueling operation and after the delivery of the voucher, unless this is prevented by automatic blocking devices. To avoid such misuse, a blocking circuit has been provided in self-service filling pumps, which is released upon replacement of the hose nozzle and can be unblocked again only by a special measure, e.g. by operating a switch at the cashier's desk. To this end, it is necessary to lay a special control line from the control desk, which is located in the filling-station building and also contains the indicatorlamps, to the filling pump. Since, in general, this has to be done on installed filling pumps, this entails considerable expense. To simplify this problem, a solution has been proposed wherein one of the three phase conductors of the three-phase system of the fuel pump is used as the control line. Via this control line a commercially available switching relay is controlled by means of which the individual switching stages are unblocked or blocked in the manner of a Geneva stop. It has turned out, however, that such control devices are highly susceptible to trouble, particularly since the coils of the switching relays are constantly subjected to a high voltage. A consequence of this susceptibility to trouble is unreliable operation. For example, no defined unblocked or blocked position is obtained, quite apart from the dependence of the electromechanical relays on vibrations and the like.